Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The sorption of Np(V) on Na-montmorillonite (STx-1) has been studied in the absence of inorganic carbon and under air-equilibrated conditions. Batch experiments were performed with 0.1 and 0.01 M NaClO4 as background electrolyte, 8 × 10-12 and 9 × 10-6 M Np(V), and 3 ≤ pH ≤ 10. At pH > 8 the presence of inorganic carbon has a strong influence on the sorption behavior of Np(V) due to the formation of aqueous Np(V) complexes with carbonate. Neptunium LIII-edge extended X-ray absorption fine structure (EXAFS) measurements on Np(V)/montmorillonite samples with Np(V) loadings in the range of 0.3-3.5 μmol/g have been performed to determine the local structure of Np at the solid-liquid interface. Wet paste samples were prepared at pH 9.0 and 9.5 in the absence and presence of inorganic carbon. The EXAFS spectra of samples prepared under ambient air conditions (pCO2 = 10-3.5 atm) revealed the formation of Np(V)-carbonate complexes at the montmorillonite surface.
The results of the batch experiments obtained under CO2-free conditions could be modeled using the two site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) model described in [1]. For modeling the sorption behavior of Np(V) on montmorillonite in the air-equilibrated system, the aqueous complexation of Np(V) with carbonate [2] was included and the following additional surface complexation reaction was required: ≡SOH + NpO2 + + CO3 2- ⇔ ≡SONpO2CO3 2- + H+. This study, combining batch experiments, spectroscopic measurements, and surface complexation modeling contributes towards a better understanding of the sorption of neptunium in the near field (bentonite) and far field (argillaceous rocks) of nuclear waste repositories.

[1] M.H. Bradbury, B. Baeyens, Modelling the sorption of Mn(II), Co(II), Ni(II), Zn(II),
Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) on montmorillonite: Linear free
energy relationships and estimates of surface binding constants for some selected heavy metals
and actinides, Geochim. Cosmochim. Acta 69, 875-892, 2005.
[2] Chemical Thermodynamics of Neptunium and Plutonium, (Eds. J. Fuger et al.) Elsevier,
Amsterdam 2001.